基于裂缝宽度的部分预应力混凝土梁设计方法

为了简化部分预应力混凝土梁的设计过程, 减少设计试算的次数, 缩小预应力筋用量的取值范围, 提出了基于裂缝宽度的部分预应力混凝土梁设计方法; 从正常使用状态的裂缝宽度出发, 根据《公路钢筋混凝土及预应力混凝土桥涵设计规范》 (JTG D62—2004) (简称《公路规范》) 中对裂缝宽度的规定, 通过最大裂缝宽度求解受拉区普通钢筋的应力, 并建立关于开裂截面中性轴高度的一元三次方程; 根据预应力筋的有效应变要求, 结合《公路规范》中最小配筋率的规定, 得到了预应力筋用量的上、下限; 给出了设计方法的主要步骤和具体验算过程, 并设计了1根T形截面试验梁, 以验证设计方法的合理性。研究结果表明: 验算梁的抗弯承载力及预应力筋用量的上、下限满足规范要求; 试验梁的荷载与挠度基本呈现三折线关系, 在外荷载为50.0kN时, 试验梁跨中出现裂缝, 外荷载为128.5kN时, 试验梁受拉普通钢筋屈服, 外荷载为157.8kN时, 试验梁跨中混凝土压碎破坏, 试验梁总体呈延性破坏特征, 满足承载性能要求; 在受拉普通钢筋屈服前, 试验梁实测最大裂缝宽度为0.18mm, 未超过预估的最大裂缝宽度0.20mm, 满足正常使用要求。可见, 提出的设计方法合理、可行, 能够简化部分预应力混凝土梁的设计过程。 

Design method of partially prestressed concrete beam based on crack width

In order to simplify the design process of partially prestressed concrete (PPC) beam and reduce the number of trial calculation and the range of prestressed reinforcement amount, a design method of partially prestressed concrete (PPC) beam based on crack width was proposed.Starting from the crack width under normal service condition and based on the regulations of crack width in Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts (JTG D62—2004) (Highway Code for short), the stress of non-prestressed reinforcement in tensile region was derived based on the maximum crack width.A cubic equation was established to calculate the neutral axis height of cracking section.Then based on the requirements of effective strain of prestressed reinforcement and the provisions of the minimum reinforcement ratio in Highway Code, the maximum and minimum amounts of prestressed reinforcement were obtained.Key design procedure and a detailed design example were also presented.Furthermore, a T-shaped test girder was designed to verify the design proposed method.Research result shows that the flexural bearing capacity and the maximum and minimumamount of prestressed reinforcement of designed girder can meet the code requirements.The load and deflection of test girder basically show a tri-line relationship.When the external load is50.0 kN, crack occurs in the midspan of test girder.When the external load is 128.5 kN, the tensile non-prestressed reinforcement yields.When the external load is 157.8 kN, the concrete is broken in the midspan of test girder.The test girder designed by the proposed method exhibits the characteristics of ductile failure and can meet the requirements of bearing performance.The measured maximum crack width of test girder is 0.18 mm before the non-prestressed reinforcement yielding and less than the predicted maximum crack width 0.20 mm, which means that it can meet the requirements of normal use.Obviously, the proposed design method is reasonable and feasible for practical design.It can simplify the design procedure of PPC beam.9 figs, 25 refs.